Correct dask and bokeh dependencies

[ianthomas23]

Fixes #1053.

install_requires in setup.py now contains dask, which is what pip requires. conda requires dask-core only which is what the pyctdev mapping in setup.cfg produces. These changes ensure that bokeh is not an install dependency for datashader.

The conda-forge datashader-feedstock will need fixing in line with this, but I will do that when 0.14.2 is released and the conda-forge bot identifies the new release and raises a PR for the updates.

I have also changed license_file to license_files as the former is deprecated, and added spatialpandas to the examples dependencies as example 8 (polygons) uses spatialpandas. There is no requirement to add the full dask to the examples dependencies as this is automatically pulled in by the spatialpandas requirements.

spatialpandas needs to have similar changes to this PR, and we will have to check how the dask dependencies chain through the libraries following that. But there will be a datashader release before the next spatialpandas release and this PR is sufficient until the latter occurs.

I have check local installs using pip and conda (via pyctdev) and also built both pip and conda packages locally to check that the changes here are correct.

[jbednar]

Sounds good, thanks!

[ianthomas23]

Confirmed that conda install -c pyviz/label/dev datashader today gives you datashader 0.14.2 with dask-core and without bokeh.

[ianthomas23]

Also confirmed that conda install -c conda-forge datashader today gives you datashader 0.14.2 with dask-core and without bokeh.